fix: make router socket non-blocking to prevent poll loop stalls

sketch/transport.h

@@ -0,0 +1,28 @@
+/*
+    This file is part of the Arduino_RPClite library.
+
+    Copyright (c) 2025 Arduino SA
+
+    This Source Code Form is subject to the terms of the Mozilla Public
+    License, v. 2.0. If a copy of the MPL was not distributed with this
+    file, You can obtain one at http://mozilla.org/MPL/2.0/.
+    
+*/
+
+#ifndef RPCLITE_TRANSPORT_H
+#define RPCLITE_TRANSPORT_H
+
+class ITransport {
+// Transport abstraction interface
+
+public:
+virtual ~ITransport() = default;
+
+virtual size_t write(const uint8_t* data, size_t size) = 0;
+    virtual size_t read(uint8_t* buffer, size_t size) = 0;
+    virtual size_t read_byte(uint8_t& r) = 0;
+    virtual bool available() = 0;
+    //virtual ~ITransport() = default;
+};
+
+#endif //RPCLITE_TRANSPORT_H